Hybrid structures have been sought for electronic and opto-electronic integrated circuits in which different portions of the circuit are composed of different materials, each having advantageous properties for the function performed in that portion. For example, GaAs offers very high speed electronic operation and opto-electronic properties in a useful wavelength range. However, GaAs integrated-circuit technology lags significantly behind that for Si. Furthermore, GaAs substrates are substantially more expensive than Si substrates. Hence, a hybrid would be desirable having a small GaAs portion, devoted to very high-speed operation or optical functions, somehow formed on a Si substrate also including a complex Si integrated circuit portion. Such a hybrid is even more desirable for InP or InGaAs opto-electronic circuits useful in the infrared spectrum because of the relative immaturity of the InP technology.
Except in a few special cases of lattice-matched materials, direct epitaxial growth of a first material on a substrate of a second material results in a hetero-epitaxial layer of the first material having much poorer structural, optical, and electrical properties than if the epilayer were grown on a substrate of the same material. For example, GaAs directly grown on a Si substrate contains at least 10.sup.6 dislocations per square centimeter because of the 4% lattice mismatch and the large difference in thermal expansion coefficients between the two materials. Elaborate techniques can somewhat reduce the defect level, but lasers and minority-carrier devices built in such heteroepitaxial structures have demonstrated poor quality and limited lifetimes.
Several techniques exist to fusion bond a semiconductor material to another material. These have involved high pressure, high temperature, or an oxide-reducing atmosphere. If a semiconducting thin film is desired, it has been typical to bond a semiconducting wafer to the other material and then thin the wafer down to thin-film thickness.
Gmitter et al. have disclosed an epitaxial lift-off and bonding procedure in U.S. Pat. Nos. 4,846,931 and 4,883,561. They first produce a free-standing thin film of GaAs or similar material by selective etching of an underlying AlAs layer. This thin film is then bonded to an almost arbitrary substrate by Van der Waals force. The flexibility of the thin film allowed its bonding to the locally varying topography of a typical substrate. However, the Van der Waals bonding has not been as robust as desired, and the bonded films have shown a tendency to peel away from the substrate under severe conditions, such as later high-temperature processing. A Van der Waals bond is inherently weaker than a chemical bond. Cross-section micrographs of the Van der Waals bond have shown that the bonding layer consists of 2 to 10 nm of native oxide of the semiconductor. Furthermore, the bonding has necessarily involved the planar bonding of an entire thin-film segment to the substrate.
Palladium is known to form good ohmic contacts with heavily doped GaAs by dispersing the GaAs oxide and reacting with the GaAs, as described, for example, by Lau et al. in U.S. Pat. No. 5,045,502. However, the palladium-based ohmic contact is then overlaid with a metal lead. As a result, the metal lead is deposited as a thin film on the palladium and is not bonded to it as a free-standing body.